The deal marks an attempt by Facebook to maintain dominance in an area that has proved one of its stickiest and most popular features: photo sharing. At the end of last year, Facebook users were uploading more than 250 million photos a day, on average, according to Facebook's filing with the SEC. The social network's members also spend nearly a fifth of their time on the site browsing photos, a 2011 comScore study found.

Instagram has, in just two short years, developed an offering that has proved attractive and compelling to a diverse group of users, and Facebook no doubt hopes to capitalize on the photo sharing app's expertise, particularly in mobile.

"For years, we've focused on building the best experience for sharing photos with your friends and family," the Facebook CEO explained. "Now, we'll be able to work even more closely with the Instagram team to also offer the best experiences for sharing beautiful mobile photos with people based on your interests."

Zuckerberg also wrote, "We will try to learn from Instagram's experience to build similar features into our other products. At the same time, we will try to help Instagram continue to grow by using Facebook's strong engineering team and infrastructure."